How big should a run in shed be for one horse?
The size of a run-in shed for one horse should be about 16 feet deep. Every horse's "personal" space is different therefore the larger the shed, the better. You can get more information at http://myhorse.com/blogs/barns-farms-ranches/build-your-horse-a-run-in-shed-with-good-planning/.
Well my 3 horses(2 geldings and a stallion) like to share 1 space especially when it's cold. They go EVERYWHERE together LoL. It's up to what is comfortable for your horse
For them to comfortably lay down, turn around,etc 12x12 is a good size.